I. Field of the Invention
The present invention relates to an automatic guidance system for an unmanned vehicle wherein the unmanned vehicle travels along a course without guide members to a destination.
II. Description of the Prior Art
Several conventional unmanned vehicles traveling along a course without guide rails have been proposed. In these unmanned vehicles, an angular velocity of the running vehicle is detected by a gyro or a rotational angle of a wheel for causing the vehicle to travel along a running path. A position error of the vehicle with respect to the moving path is calculated. The vehicle is then automatically steered to correct the position error.
There are two types of conventional automatic guidance systems for an unmanned vehicle. In a first conventional automatic guidance system, a position error is directly calculated in accordance with information such as the rotational angle of the vehicle wheel or the angular velocity detected by a gyro, and the position error is corrected. In a second conventional automatic guidance system, the actual position and heading angle of an unmanned vehicle are measured and are compared with those for a predetermined path, and the actual position error is calculated and corrected.
According to the first automatic guidance system, driving means control values such as a steering angle or azimuth angular velocity to be given to the unmanned vehicle in accordance with a moving distance are stored in a memory means as path information for automatically driving the unmanned vehicle. By referring to the storage contents of the memory means in accordance with a distance calculated by a rotational angle of the wheel, the readout steering angle or azimuth angular velocity is supplied to the driving means, so that the driving means is driven in accordance with these pieces of information. At the same time, the detected azimuth angular velocity information is subtracted from the readout steering angle or azimuth angular velocity information to calculate a position error. A correction signal for correcting the position error is calculated in accordance with a predetermined mathematical expression and is superposed on the readout values. The resultant information corrects the position error so that the unmanned vehicle can be automatically driven.
According to this automatic guidance system, the steering angle and azimuth angular velocity (i.e., a steering value) which correspond to a moving distance are stored as motion data in the memory means, and the memory means must be mounted in the unmanned vehicle. However, when a complicated course is included in a course network having many intersections, the necessary steering value information for the unmanned vehicle must be prepared for all moving courses, so that a great amount of data having high redundancy must be used. Therefore, the memory cannot be effectively used, resulting in inconvenience. In addition to this disadvantage, it is laborious to create these data. Therefore, the first conventional automatic guidance system can be applied only to a relatively simple moving course.
Furthermore, when a preset moving course is partially modified to obtain another moving course, a moving distance up to a point where the course is modified must be calculated, and all steering values for the remaining distance must be modified in accordance with the modified course. In practice, it is difficult to process such a great amount of data in a short period of time. This conventional system cannot be applied to a flexible unmanned vehicle which changes its moving course in accordance with surrounding conditions.
According to this conventional automatic guidance system, correction of the position error is performed in accordance with a difference between the preset azimuth angular velocity and the detected azimuth angular velocity or between the preset steering angle and the detected steering angle. The position error at the initial period of travel of the unmanned vehicle cannot be corrected, and changes in moving distance up to the destination are caused by zig-zag motion and cannot be corrected. Then, guidance becomes inaccurate in principle. In addition to this disadvantage, the position error representing a deviation from the moving path cannot be predicted for correction before the position error occurs. The correction of the position error is delayed, thus disabling preventing stable motion.
In view of the drawbacks of the first conventional automatic guidance system for controlling the vehicle motion in accordance with steering value information, the second conventional automatic guidance system has been proposed. According to this system, the position and the heading angle of the unmanned vehicle are calculated once in accordance with rotational angle information of the wheel or the like, and the vehicle is controlled in accordance with this information. A dot array represented by a coordinate system and plotted on a road surface is stored as moving path data of the unmanned vehicle. A moving course represented by a line obtained such that dots are connected with reference to the dot array is compared with the calculated position and heading angle information. When the vehicle is located to the left of the course, the vehicle is driven to the right. However, when the vehicle is located to the right of the course, the vehicle is driven to the left. As a result, the vehicle is driven along the course. According to this system, the position error of the vehicle can be corrected from the very beginning of travel. The motion data is not given by the moving distance but by the position and the heading angle. Even if the vehicle is driven in a zig-zag manner, the vehicle can be accurately guided to the destination. However, in order to cause the unmanned vehicle to accurately travel along a curved path, dot array data is increased and a memory having a large capacity is required. According to this automatic guidance system, the moving speed information can be used in addition to the position information and the heading angle information so as to predict a position error representing a deviation from the path in the near future. Therefore, position error correction is performed at a prediction point prior to actual occurrence of the position error, and more stable control can be achieved. However, it is very difficult to calculate a control value for correcting the position error. When the vehicle greatly deviates from the path in the system wherein the control value is set in the driving means, the vehicle is kept steered to the right or left and traces a circular locus and cannot return to the proper course. In addition, in a right-angled path, special operations are required. The deviation from the path is controlled independently of the moving speed, so the vehicle turns a right-angled corner with a larger radius of curvature, resulting in inconvenience. While the unmanned vehicle is being guided, the vehicle is inevitably stopped. However, it is very difficult to stop the vehicle in accordance with the conventional control operation of the deviation from the path performed independently of the moving speed. Even the second conventional automatic guidance system still remains in the experimental stage.
According to the conventional techniques for guiding an unmanned vehicle driven along a course without guide rails or the like, a memory device for storing a great amount of data must be mounted in the unmanned vehicle to cause the vehicle to travel a complicated course network. The system as a whole becomes expensive, and the vehicle cannot be stopped at a destination. A practical model has not yet been developed.